The present invention relates generally to electronically commutated DC motors (i.e., brushless DC motors) and, more particularly, to a method and system of improved closed loop control of sensorless brushless DC motors.
Brushless direct current motors are well known in the art. The phase windings therein are sequentially energized at appropriate times so as to produce a rotating magnetic field relative to a permanent magnet rotor. The timing of this sequential energization is a function of the location of the permanent magnetic rotor with respect to the particular phase winding that is to be energized. Various means have been heretofore used to sense the position of the permanent magnet rotor relative to the phase windings. These have included optical sensors and Hall effect devices that feed a position signal to switching logic that selectively switches power on and off to the respective phase windings. However, such sensing devices add cost and complexity to a system, and may moreover require maintenance from time to time to assure continued proper operation. In certain high flux/power applications, such as those employing 350 volt motors, the Hall sensors are a common point of failure.
Thus, as a result of the drawbacks of sensor devices, attention has also been focused on sensorless systems that are not based on any direct sensing of the rotor position itself. Generally speaking, sensorless systems are used to measure the effect of the back electromotive forces (BEMF) produced in the energized windings by a rotating rotor. For example, in a trapezoidal three-phase brushless DC motor (BLDC), the phase currents are applied in a bipolar fashion; that is, while two of the three phases are driven, the other is de-energized. The transition of a phase winding to a neutral point determines the instant in time when (30 electrical degrees later) the control circuitry energizes the next pair. Accordingly, the control circuitry of a sensorless BLDC motor must have information regarding the neutral voltage of the motor phase windings.
The neutral voltage of a three-phase motor may be measured either directly or indirectly. For a WYE wound motor, a center tap can be directly connected to the neutral point of the motor windings (i.e., the common point of the three phase coils schematically arranged similar to the letter xe2x80x9cYxe2x80x9d). However, for a delta wound motor, there is no neutral point since the phase windings are configured in a triangle arrangement. Thus, an indirect method of measuring the neutral voltage is generally preferred, since it is applicable to either delta or WYE wound motors, and does not result in additional costs in manufacturing additional center taps for the WYE motors. Such an indirect method takes advantage of the fact that, at any given instant in time, the neutral voltage of a brushless machine with trapezoidal BEMF is the sum of the three phase voltages divided by three. As such, the system need only sense the voltages across each of the three phase windings to indirectly determine the neutral voltage
One shortcoming of this BEMF sensing technique, however, stems from the fact that the BEMF is directly proportional to the motor speed. Once the motor reaches a sufficient speed, the generated BEMF will be of sufficient magnitude to be detected for closed loop control of the motor. Prior to that time, conventional sensorless motor drives typically accelerate the motor in an open loop mode, wherein the commutation signals are applied at a rate designed to approximate the acceleration characteristics of a given motor/load combination. Unfortunately, the inability to precisely detect BEMF values at low speeds can lead to rotor position inaccuracies, and possibly even loss of synchronization. Accordingly, it is desirable to be able to accurately commutate a brushless machine in a closed loop mode at relatively low speeds.
The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method for electronically commutating a motor having a plurality of phase windings associated therewith. In an exemplary embodiment, the method includes sensing a back electromotive force (BEMF) generated by each of the phase windings, and scaling the magnitude of the sensed BEMF values for each of the phase windings to a normalized value to produce gain corrected BEMF signals. The gain corrected BEMF signals are then used to determine a rotor position of the motor. In a preferred embodiment, an automatic gain control circuit is configured for scaling the magnitude of the sensed BEMF values. The normalized value is based upon about half the value of a DC bus voltage of the motor.
In another aspect, a control circuit for a sensorless brushless motor includes a controller for receiving a sensed back electromotive force (BEMF) generated by each of a plurality of phase windings of the motor. An automatic gain control circuit is used for scaling the magnitude of sensed BEMF values for each of the phase windings to a normalized value to produce gain corrected BEMF signals. The gain corrected BEMF signals are used by the controller to determine a rotor position of the motor.
In yet another aspect, a sensorless brushless motor includes a plurality of phase windings energized by a direct current source. An inverter sequentially applies phase current from the direct current source through selected pairs of phase windings. A controller receives a sensed back electromotive force (BEMF) generated by each of the plurality of phase windings of the motor, the controller providing corresponding control signals to control switching of the inverter. In addition, an automatic gain control circuit is used for scaling the magnitude of sensed BEMF values for each of the plurality of phase windings to a normalized value to produce gain corrected BEMF signals. The corrected BEMF signals are used by the controller to determine a rotor position of the motor.